Category (category theory)

Revision as of 22:32, 2 September 2008 by Jam (talk | contribs) (discrete and opposite categories)

A category, $\mathcal{C}$, is a mathematical object consisting of:

  • A class, $\text{Ob}(\mathcal{C})$ of objects.
  • For every pair of objects $A,B\in \text{Ob}(\mathcal{C})$, a class $\text{Hom}(A,B)$ of morphisms from $A$ to $B$. (We sometimes write $f:A \to B$ to mean $f\in \text{Hom}(A,B)$.)
  • For every three objects, $A,B,C \in \mathcal{C}$, a binary operation $\circ: \text{Hom}(B,C) \times \text{Hom}(A,B) \to \text{Hom}(A,C)$ called composition, which satisfies:
    • (associativity) Given $f:A\to B$, $g:B\to C$ and $h:C \to D$ we have \[h\circ(g\circ f) = (h \circ g)\circ f.\]
    • (identity) For and object $X$, there is an identity morphism $1_X:X\to X$ such that for any $f:A\to B$: \[1_B\circ f = f = f\circ 1_A.\]

The class of all morphisms of $\mathcal{C}$ is denoted $\text{Hom}(\mathcal{C})$.

A category $\mathcal{C}$ is called small if both $\text{Ob}(\mathcal{C})$ and $\text{Hom}(\mathcal{C})$ are sets. If $\mathcal{C}$ is not small, then it is called large. $\mathcal{C}$ is called locally small if $\text{Hom}(A,B)$ is a set for all $A,B\in \text{Ob}(\mathcal{C})$. Most important categories in math are not small, but are locally small.

Intuitively we can think of the objects of $\mathcal{C}$ as being sets (perhaps with some additional structure) and morphisms as being functions between these sets (perhaps satisfying some properties) and composition as being regular function composition, however there are examples of categories which do not satisfy this. Typically when studying category theory we deal with morphisms and composition completely abstractly (similarly to how we study multiplication abstractly in group theory), and never talk about 'plugging things in to' morphisms.


Some common examples of categories are:

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